U.S. Pat. No. 6,234,645 describes a system comprising at least four light-emitting diodes for the production of white light. The colour rendering index is above 60 and the luminous efficacy is preferably above 30 lm/W. In one embodiment, the colour temperature of the light can be adjusted by selectively switching the light-emitting diodes.
U.S. Pat. No. 5,783,909 describes a system for maintaining the luminous intensity of the light from at least one light-emitting diode. The system comprises a power supply electrically connected to the light-emitting diode for supplying pulses of electrical energy to this light-emitting diode. By adjusting the electrical energy supplied the luminous intensity of the light-emitting diode can be maintained at a pre-selected level, thereby compensating for the diminution of the output due to e.g. temperature variations or ageing.
U.S. Pat. No. 6,012,291 describes a system for temperature control of an optical semiconductor device, e.g. a light-emitting diode. By attaching the semiconductor device to a thermal conductor the temperature of this optical semiconductor device is kept at a constant temperature level despite any influence of the ambient temperature, heat sources etc.